Long wavelength (far infrared) radiation heating device



1968 KOSAKU ISOBE ETAL 3,413,441

LONG WAVELENGTH (FAR INFRARED) RADIATION HEATING DEVICE 2 Sheets-Sheet 1 Filed Nov. 22, 1965 26, 1963 KOSAKU ISOBE ETAL 3,413,441

LONG WAVELENGTH (FAR INFRARED) RADIATION HEATING DEVICE Filed Nov. 22, 1965 2 Sheets-Sheet 2 HEATING ELEMENT United States Patent 3,413,441 LONG WAVELENGTH (FAR INFRARED) RADIATION HEATING DEVICE Kosaku Isobe, Hajime Iinuma, Ynkio Tanaka, Katsuji Soya, and Hitoshi Yamagishi, Tokyo-to, Japan, assignors to Kokusai Denki Kabushiki Kaisha (also known as Kokusai Electric C0., Ltd.), Tokyo-to, Japan, a jointstock company of Japan Filed Nov. 22, 1965, Ser. No. 508,941 Claims priority, application Japan, Dec. 14, 1964, 39/ 70,018 3 Claims. (Cl. 219390) This invention relates to techniques in heating various substances by radiation and more particularly to heating devices of the type having a hollow cylindrical heating element adapted to heat therein articles to be heated with radiation rays.

More specifically, the invention concerns a new radiation device of the above stated type, one unique feature of which is that, in order to convert, with high efliciency, input electrical energy into radiant energy, the in ner surface of the hollow cylindrical heating element is coated with a powder layer of high emissivity such as, for example, ferric oxide (Fe O and the outer surface of the heating element is coated with a powder layer of low emissivity such as, for example, aluminium oxide By the provision of these coatings, long wavelength infrared rays are radiated toward the article to be heated, and, at the same time, short wavelength infrared rays are radiated toward a hollow cylindrical reflectordisposed concentrically around the heating element.

In general, the radiation absorption characteristic of a substance is good in the radiation band of long wavelength infrared rays. Accordingly, radiation of this wave length band is desirable at the inner side of the heating element, and for increasing the effectiveness of the reflector, the radiation band of short wavelength infrared rays is, of course, desirable. These physical properties are advantageously utilized in the present invention.

.It is an object of the present invention to provide a long wavelength radiation heating device having substantially high efliciency, that is, having a heating efficiency comparable to that in the case low-frequency induction heating.

Another object of the present invention is to provide a heating device of the above stated character which is of simple and relatively inexpensive construction and operation.

According to the present invention, briefly stated, there is provided a long wavelength (far infrared ray) radiation heating device having a hollow cylindrical heating element adapted to heat, within its hollow interior, articles to be heated with long wavelength infrared radiation rays and power supply means connected to the heating element to supply electrical power thereto, said radiation heating device being characterized by an outer hollow cylinder disposed concentrically around the heating element and having low emissivity and excellent reflecting characteristic with respect to short wavelength (near infrared) radiation rays, said cylindrical heating element being coated over its entire inner surface with a first substance having high emissivity and long wavelength (far infrared) radiation characteristics and over its entire outer surface with a second substance having low emissivity and short wavelength (near infrared) radiation characteristics.

The nature, principle, and details of the invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accomtive material having a suitable resistivity and has a longitudinal slot 6 completely through its cylindrical wall, the power source S being connected to opposite edges of the slot 6. By using a heating element of this construction with an inner diameter of 183 mm. and length of 420 mm., depositing a coating of Fe O powder on its inner surface 2 and a coating of A1 0 powder on its outer surface 3, and operating the heating element 1 by applying thereto power of 100 volts and 280 amperes, it is possible to obtain a surface temperature of approximately 1,200 degrees C.

Under these conditions, the Fe O powder coating radiates long wavelength rays (far infrared rays) of from 8a to 251.1. or longer in directions perpendicular to the heating element axis X-Y (as indicated by arrow P) with high emissivity of the order of 61 0.9- In an actual instance when a brass billet of 100 mm. diameter and 400 mm. length was placed as an article '5 to be heated at the center of the heating element 1, this billet was heated uniformly to 700 degrees C. in 11 minutes. In this manner, long wavelength radiation heating of billets can be accomplished in a practical manner, and, accordingly, by using this heating device in conjunction with an extruding machine, for example, it is possible to carry out uniform heating suitable for producing products such as brass bars and brass tubes.

On the other hand, the A1 0 powder coating deposited on the outer surface 3 of the heating element 1 radiates short wavelength (near infrared) rays of approximately from 0.7,u. to 3,. We have found that the emissivity e of this radiation is only of the order of 6220.2-

We have found further that, as a result in the case of the above described example, approximately 20 percent of the heating rate produced by 28 kw. (100* volts 280 amperes), or 5.6 kw., is heat transfer loss (including convection loss), and the remaining percent, or 22.4 kw., is radiated as radiant rays,

0.9 22.41010. X m 18.3kw.

of which is radiated inwardly, and

of which is radiated outwardly.

When the distance between the heating element 1 (inner cylinder) and the outer cylinder 4 is 50 mm., and the outer cylinder 4 is constructed from aluminium sheet of 1 mm. thickness, this outer cylinder has a reflecting power of approximately percent with respect to radiation rays of short wavelength within 3a. Accordingly, the outer cylinder functions as a reflector and, upon receiving radiation heat at the rate of 4.1 kw., reaches an equilibrium state at a temperature of approximately degrees C.

However, if the wavelength of the radiation energy of this 4.1 kw. is of a long wavelength of 3a or longer, the reflecting power of the aluminium sheet will drop to 70 percent or lower, and the temperature rise of the outer cylinder will increase.

Thus, the article 5 to be heated is subjected to radiation heating by long wavelength (for infrared) rays of 22.4kw. 24.11am

3 18.3 kw. We have found that the absorption rate of the heated article 5 in the case of long wavelength (for infrared) rays of 8,1. or longer is 1.7 or more times that in the case of short wavelength (near infrared) rays of 3 or less.

The heating efliciency n in this case becomes whereby it is apparent that a heating operation comparable to that by low-frequency induction heating can be practically achieved. Moreover, the percent ratio of the quantity of heat received by this brass billet in being uniformly heated at 700 degrees C. and the input electric power, that is, the overall efliciency, is 37 percent.

In order to indicate still more fully the nature and utility of the present invention, the following examples of tests and results are set forth, it being understood that these examples are presented as illustrative only, and that they are not intended to limit the scope of the invention or of its applicability.

Specimen billets of the same dimensions of aluminium, brass, steel, and copper were heated uniformly at temperatures of 500, 700, 630, and 800 degrees C., respectively, by means of heating device according to the invention whereupon the following results were obtained.

X 1002181 percent Billet Heating Tem- Heating Time Overall efiiperature C.) (m1n.)(sec.) ciency (percent) Aluminium 500 7' 38 Brass 700 11 37 Steel 630 1030 48 Copper 800 1140 47 Aluminium billets of different diameters and the same length of 600 mm. were heated, whereupon the following overall efliciencies were obtained.

Al billet: Overall eff., percent 125 mm. diameter 52 100 mm. diameter 38 80 mm. diameter 29 Billet Heating tem- Heating time Overall efli- (100 mm. diam., perature C.) (min.)(see.) ciency (percent) 400 mm. length) Aluminium. 500 1312" 10 Brass.... 700 42 5 Ste 630 32 8 Comparison of these results with the results obtained with the heating device according to the invention indicates that the coatings have a great effect in increasing the overall heating efliciency.

During these tests, it was found that the outer cylinder (reflector) made of aluminium sheet was heated to a temperature exceeding 500 degrees C. and was beginning to soften. This indicates that the alumina coating on the outer surface of the heating element has a great effect in suppressing radiation loss therefrom. It was found further that, since the Fe O and A1 0 powders are bonded and secured with water glass (Na SiO the coatings of these powders do not peel in spite of the repetitions of heating and cooling of the heating element.

A difference of approximately degrees C. was indicated between the temperatures of the heating element with and without the Fe O powder coating. This indicates that, by applying the coating of Fe O powder, the temperature of the heating element is lowered by 80 degrees, that is, the emissivity thereof is increased. This means that, as an intrinsic characteristic of Fe O its radiation wavelength characteristic naturally produces high emissivity in the long wavelength band, and the absorption at the metal surface is excellent.

While the coating of the heating element with A1 0 powder has a great effect in reducing the emissivity thereof, its radiation wavelength distribution is displaced greatly toward the short wavelength band in comparison with that of Fe O and also has the function of increasing the reflecting efficiency at the surface of the aluminium sheet.

In FIG. 2, there is shown a modification of the above described embodiment of the invention, in which the cylindrical heating element 11 having an inner surface 12 and an outer surface 13 is fabricated by winding an electrical resistance heating ribbon of 0.8 mm. thickness and 32 mm. width into a helical coil of 183 mm. inner diameter with a spacing of 3 mm. providing an air gap 16 between adjacent turns of the ribbon and a total of 12 winding turns and is supported inside the outer cylinder 14 on 6 supporting bars. The object 15 to be heated is positioned at the center of the heating element.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and a modification thereof and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention as set forth in the appended claims.

What we claim is:

1. A long wavelength radiation heating device comprising a hollow cylindrical heating element adapted to heat within its hollow interior articles to be heated with long wavelength infrared radiation rays, power supply means connected to the heating element to supply electrical power thereto, and an outer hollow cylinder disposed concentrically around the heating element and having low emissivity and excellent reflecting characteristic with respect to short wavelength radiation rays, said cylindrical heating element being coated over its entire inner surface with a first substance having high emissivity and long wavelength radiation characteristics and over its entire outer surface with a second substance having low emissivity and short wavelength radiation characteristics.

2. The radiation heating device as claimed in claim 1, wherein: the heating element is made of an electric resistance heating material; said first and second substances are, respectively, powders of ferric oxide and aluminium oxide; and the outer cylinder is formed from aluminium sheet.

3. The radiation heating device as claimed in claim 1, wherein the heating element consists of an electric resistance heating ribbon wound into a helical coil of hollow cylindrical form with a certain air gap between adjacent winding turns.

References Cited UNITED STATES PATENTS 2,848,591 8/1958 Taylor 219-390 X 3,146,337 8/1964 Ieru 219406 X 3,257,492 6/1966 Westeren 2l9406 X BERNARD A. GILHEANY, Primary Examiner.

R. N. ENVALL, Assistant Examiner. 

1. A LONG WAVELENGTH RADIATION HEATING DEVICE COMPRISING A HOLLOW CYLINDRICAL HEATING ELEMENT ADAPTED TO HEAT WITHIN ITS HOLLOW INTERIOR ARTICLES TO BE HEATED WITH LONG WAVELENGTH INFRARED RADIATION RAYS, POWER SUPPLY MEANS CONNECTED TO THE HEATING ELEMENT TO SUPPLY ELECTRICAL POWER THERETO, AND AN OUTER HOLLOW CYLINDER DISPOSED CONCENTRICALLY AROUND THE HEATING ELEMENT AND HAVING LOW EMISSIVITY AND EXCELLENT REFLECTING CHARACTERISTIC WITH RESPECT TO SHORT WAVELENGTH RADIATION RAYS, SAID CYLINDRICAL HEATING ELEMENT BEING COATED OVER ITS ENTIRE INNER SURFACE WITH A FIRST SUBSTANCE HAVING HIGH EMMISSIVITY AND LONG WAVELENGTH RADIATION CHARACTERISTICS AND OVER ITS ENTIRE OUTER SURFACE WITH A SECOND SUBSTANCE HAVING LOW EMISSIVITY AND SHORT WAVELENGTH RADIATION CHARACTERISTICS. 